1. Field of the Invention
The present invention relates to a nonaqueous solvent and a nonaqueous electrolytic solution usable for an electricity storage device for storing or accumulating electrochemical energy, and a nonaqueous electricity storage device using the same, such as a lithium secondary battery or an electrical double layer capacitor.
2. Description of the Related Art
Recently, high voltage electricity storage devices in which a charge voltage and a discharge voltage of a single electrical storage element each exceeds 1.5 V have been progressively developed. As such high voltage electricity storage devices, lithium primary batteries, lithium secondary batteries, lithium polymer secondary batteries, electrical double layer capacitors and the like have been put into practice.
For a high voltage electricity storage device, a nonaqueous electrolytic solution using an organic compound as a solvent is used. The reason for this is that water, if used as the solvent of the electrolytic solution, is electrolyzed by the high charge voltage and discharge voltage. A nonaqueous electrolytic solution is also used for an electricity storage device including an electrode which contains active lithium reactive with water and uses occlusion or release of lithium.
The nonaqueous electrolytic solution is desired to have a high conductivity and a low viscosity in order to improve the discharge performance of the electricity storage device in which the nonaqueous electrolytic solution is used. When used as a solvent for a secondary battery, an electrical double layer capacitor or the like, the nonaqueous electrolytic solution needs to be stable chemically and electrochemically in order not to deteriorate the performance of the electricity storage device as a result of the repetition of charge/discharge cycles.
From these points of view, for example, as a main solvent of the electrolytic solution of a lithium ion secondary battery, a mixed system of a cyclic carbonate (carbonic acid ester) represented by ethylene carbonate and a chain carbonate represented by ethylmethyl carbonate or dimethyl carbonate is conventionally used. As a main solvent of the electrolytic solution of an electrical double layer capacitor, a cyclic carbonate represented by propylene carbonate is preferably used. The above-described electricity storage devices are widely used as a main power source, a backup power source or an electric circuit power source for mobile communication devices or portable electronic devices. Such mobile and portable devices are recently desired to be more compact and to have higher performance. Thus, the electricity storage devices are required to be further improved in the volumetric energy density.
In order to improve the volumetric energy density, it is necessary to improve the average discharge voltage and the volumetric capacity density. As one measure for realizing this, it is proposed to increase the charge voltage.
In the case of a lithium ion secondary battery, the utilization factor of lithium as a positive electrode material can be improved by raising the charge voltage. As a result, the volumetric capacity density is increased. As the positive electrode material, a lithium-containing transition metal oxide such as lithium cobalt oxide, lithium nickel oxide or the like is generally used. In the case of an electrical double layer capacitor, the value of the electrical double layer capacity can be increased by raising the charge voltage. As a result, the volumetric capacity density can be increased.
However, in the case where one of a pair of electrodes is charged to a level equal to or higher than 4.3V on the basis of the dissolution deposition potential of lithium, the following occurs. Even when a conventional chain carbonate or cyclic carbonate which is known to be superb in oxidation resistance and so usable as a nonaqueous solvent suitable to a high voltage electricity storage device is used, such a carbonate is oxidized and decomposed to generate gas. Such a decomposition reaction conspicuously proceeds especially in a high temperature state and is accompanied by generation of a large amount of gas. Therefore, for example, where an internal-pressure-sensitive current interrupt device (CID) for blocking the charging current against excessive charge of a battery is mounted on a high voltage lithium ion secondary battery containing such a nonaqueous solvent, the CID may malfunction to cause the lithium ion secondary battery to lose the function as a battery. In the case where the CID is not mounted, a problem arises that when the amount of gas generation is excessive, the battery is swollen.
Japanese Laid-Open Patent Publication No. 2005-149750 discloses a nonaqueous electrolytic secondary battery using a nonaqueous electrolytic solution containing cyclic sulfonic acid ester in order to suppress the chain carbonate or the cyclic carbonate from being oxidized and decomposed at a super high potential. In such a nonaqueous electrolytic secondary battery, when the positive electrode is charged to a potential equal to or higher than 4.5 V, the cyclic sulfonic acid ester is oxidized and decomposed on the positive electrode, and a surface of the positive electrode is covered with a film. This film suppresses the solvent from being decomposed on the surface of the positive electrode.
Japanese Laid-Open Patent Publications Nos. 2004-111359 and 2006-286650 propose incorporating, into a nonaqueous solvent, a “hydrocarbon compound which may contain a fluorine atom” at 0.01% by weight or greater and 5% by weight or less. These patent documents describe that because a hydrocarbon compound which is stable against oxidation and reduction is present at an activation point on the surface of the electrode, sub reaction of the electrolytic solution component and the electrode active material can be suppressed in a high temperature state.
The nonaqueous electrolytic secondary battery disclosed in Japanese Laid-Open Patent Publication No. 2005-149750 suppresses the decomposition reaction of the chain carbonate or the cyclic carbonate, but the effect thereof is not sufficient. In addition, because the film is formed on the surface of the positive electrode, the charge transfer resistance is increased on the interface of the positive electrode active material. This causes problems of raising the internal resistance of the battery and also reducing the high rate discharge performance.
Japanese Laid-Open Patent Publications Nos. 2004-111359 and 2006-286650 describe that the nonaqueous electrolytic secondary battery disclosed in these patent publications can suppress the sub reaction of the electrolytic solution component and the electrode active material in a high temperature state owing to a “hydrocarbon compound which may contain a fluorine atom”. However, the content of the hydrocarbon compound is as low as 5% by weight or less. In addition, the hydrocarbon compound does not have a property of, for example, adsorbing to, or coordinating at, the surface of the positive electrode. Therefore, the hydrocarbon compound does not selectively exist at a high concentration on the surface of the positive electrode. For these reasons, the nonaqueous electrolytic secondary batteries described in Japanese Laid-Open Patent Publications Nos. 2004-111359 and 2006-286650 are not considered to sufficiently provide the effect of suppressing the sub reaction.